Darkroom Printing: CAST, a powerful method for advanced curve control

[Pere Casals]

What is CAST ?


Artificial Stain Technique, inspired in the interaction between Pyro stain with the variable contrast paper.

This is only an small addition to Alan Ross selective masking, it's just looking at the same from another point of view: We make a mask to effectively deformate the paper curve to suit our desire.

Proportional stain in pyro negatives casts an stronger low contrast yellow filter over denser areas, this effetivelly modifies paper curve for highlights.

Hmmmm.... we can do the same with Ps !!! just we make a grading map to asign a color to each gray level.

... then we print colored image on a tranparency, et voila, we have a contrast mask that exposes each density level with the filter grade we selected, as always we place a mylar diffuser between the negative and mask, in that way inkjet dots in the mask are not seen !

https://live.staticflickr.com/65535/48828538282_f2a52ab863_z.jpg https://live.staticflickr.com/65535/48828538197_1b17fa862a_z.jpg


In this case we have most of the image exposed with grade 2 but highlights are exposed with 00, this mostly re-shapes paper curve to what a Silver Chloride paper would deliver.

But wait... we may make our grading map to include all color filters:


https://www.youtube.com/watch?v=FOz92KnCsHA



Important:

We may blur the original BW scan, or we may blur later the color mask... if we blur the bw image first we should work in 16-bits per channel to avoid banding. We also may not blur the mask at all, diffuser layer under the mask will do it.

[Pere Casals]

Ps work is trivial...

We depart from a blurred image, then we add a curve adjustment layer and over it the grading map:

https://live.staticflickr.com/65535/48828538112_b37e584c2d_o.jpg

The adjustment layer:

https://live.staticflickr.com/65535/48828381211_792d8f271c_o.jpg

And the grading map:

https://live.staticflickr.com/65535/48828539777_4baecb10eb_o.jpg

[Pere Casals]

Important:

If our mask includes color grades 4, 4.5 or 5 then the colors for grades from 00 to 3.5 may have to include black ink to make exposure equal, as contrast grades from 4 to 5 require x2 the exposure.


Also we may also use a two or three colors for the grading map, this would be nice to just emulate AZO curve:

https://live.staticflickr.com/65535/48828538172_5d65e51f5a_b.jpg


We also may make a bare Duotone, I started with that, but later it cannot be modified:

https://live.staticflickr.com/65535/48828539717_60ff584a6f_z.jpg

[Pere Casals]

Workflow:

1)

We may make contact prints with masks until we find a mask that delivers right curve/tonality.

Then we place the sandwich in the enlarger, with exactly same Lux and we project all that on the wall: we nail a sound monster print with little paper waste.


2) alternatively, we calibrate a process that it simply delivers a custom curve, it's like if we could customize the curve in our paper.


3) We may add grey patches to dodge some areas, as usual, etc.



______________________________


I've started to make contact prints in that way, with very good results, I guess that I need some good calibrations for the to nail the grades the Ps colors that later a the (cheapo) inkjet printer places on the transparency.

That negative in the image shown has been a sourve of frustation for me, never got the print I wanted, now I'm close.

I've solved the complex part in the image manipulation, and I simply want to improve some manual burning.

[Vaughn]

Very interesting concept, even though at first it had me thinking of The Search for the Holy Grail and the rabbit scene -- "Run away! Run away!"

[Pere Casals]

Very interesting concept, even though at first it had me thinking of The Search for the Holy Grail and the rabbit scene -- "Run away! Run away!"

Vaughn, no miracle, it only modifies the effective paper curve, overcoming a limitation we have compared to hybrid, the rest is as difficult or as easy as always it has been...

[Vaughn]

I switched to alternative processes after using just graded papers (except in late 1970s college photo classes w/ Kodak Polycontrast). I hope it works well for all!

[Jim Noel]

Very interesting concept, even though at first it had me thinking of The Search for the Holy Grail and the rabbit scene -- "Run away! Run away!"

I'll run with you!

[Larry Gebhardt]

I've tried Alan Ross's masking technique with colored filters for contrast and while it works, it's not easy. There was a lot of trial and error to get the strength of each section correct. In the end I went back to just using his pencil masking techniques for dodging and burning, and rough contrast corrections with old school dodging and burning with different filters (actually VC light mixes).

Do you have a good way to quickly make a mask that corrects contrast and density without all the trial and error?

[Pere Casals]

I've tried Alan Ross's masking technique with colored filters for contrast and while it works, it's not easy. There was a lot of trial and error to get the strength of each section correct.

Hello Larry,

Fortunately cheap cmyk inkers have a very good Magenta and a very good Yellow which are exactly the ideal colors for that, following colors in ilford filters. But we still have to balance exposure/grades.

At first I got pitfalls with the AR method in the shadows because the higher grades require x2 the exposure, so the lower grades (00 to 3.5) require a black shade that dodges 1 stops exactly. Another important thing is that in the AR mask we cannot leave a not colored area, white light requires 1 stop less, so we need to assign a color in every spot, and that color may have to be well adjusted to reproduce the curve of one of the ilford grades. If not we have weird bendings in the curves delivering an unnatural look.

Alan Ross method is absolutely sound, but you are right, not that easy, this is only a little refinement to make it way easier. As tone varies continuously across density we easily have a very smooth response delivering a natural look, so actually we depart from better situation.





Do you have a good way to quickly make a mask that corrects contrast and density without all the trial and error?

Yes ! I've two methods.

1) First is scanning alongside the ilford filters with a printed square mosaic of patches (transparency) with all levels of yellow-magenta, then we pick the patch that has the same scanned hue than the ilford filter. This is precise enough. Still if our mask has 3.5 to 5 grades then we have to dodge the other grades with black ink to conserve exposure across 00-3.5 vs 4-5 grades.

Those hues for the grades are used in the Ps grading map.


2) Proofing based on Phil Davis teaching in BTZS. Sensitometry rules the movement of starts in our firmament !
I'm finishing my freeware tool (https://www.largeformatphotography.info/forum/showthread.php?154301-Unsharp-Masks-When-Not-Worth-the-Effort&p=1518248&viewfull=1#post1518248) for paper film/calibration and mask proofing, so I calibrate the ilford grades an later I adjust colors for each grade to match the ilford curves in grade an in exposure, this has to be done only once. From that the tool predicts paper density in each spot and it makes a software proofing, WYSIWYG, still working in that, I made "proof of concept" tests to assert viability, for the moment this is promising. Now I'm busy making the curve response plots for masking maps.


_______________________


Anyway, to start with: we may make a simple mask with only 2 grades in the grading map, say a 3.5 grade for the general exposure and 00 for the highlights, with some clicks and say 3 contact copies on paper we may get a mask that solves most of complexity in the tonal manipulation. With that we still may have to burn the shadows with the 5 grade, but most would be solved.

I that case we'll place our mids more close to 3.5 or 00 depending on how we bend the curve in the adjustment layer.


I see this technique from two points of view, while with a high degree of technification we can precisely control the tonal curve in the final image we also may use that resource in a simpler way, let me explain that...

With pyro-VC interaction we modify the tonal curve in the highlights, the stronger the proportional stain the more compressed the highlights because we block more blue there. But once we have pyro developed a negative then we have a fixed stain, we are tied to that, now imagine that you can easily change the proportionalilty of the stain in the yet developed negative to adjust it for the print you want. ... so we instead place the yellow stain in a mask, so we only need to re-print the mask if we now want a diferent proportionality of the stain. This is straight to do...

I started from that: "Artificial Stain".

The rest is simple, just we apply the same concept to the rest of the density range and with other grades.


Still the Alan Ross masking is complementary to that, but the idea is to separate what we do in the tonal response from local manipulation. "CAST" modifies our "effective" tonal curve, with this done the Alan Ross selective masking is way, way easier, we may only need to also manipulate the mask for selected areas in what we may want a dedicated job.


Of course we may also leave manual dodging/burning for the paper exposure.

I don't see CAST as a "print maker" solution, while this approach is possible it also can be considered a resource to modify our effective paper curve to smoothly fit our image in the print, and a way to automate the tonal response modification in the Alan Ross selective masking, making it easier.

[RenYang]

Great tool